Two-frequency shell model for hypernuclei and meson-exchange hyperon-nucleon potentials

A two-frequency shell model is proposed for investigating the structure of hypernuclei starting with a hyperon-nucleon potential in free space. In a calculation using the folded-diagram method for Λ¹⁶O, the Λ single particle energy is found to have a saturation minimum at an oscillator frequency ħωΛ≈10MeV, for the Λ orbit, which is considerably smaller than ħωN=14MeV for the nucleon orbit. The spin-dependence parameters derived from the Nijmegen NSC89 and NSC97f potentials are similar, but both are rather different from those obtained with the Jülich-B potential. The ΛNN three-body interactions induced by ΛN-ΣN transitions are important for the spin parameters, but relatively unimportant for the low-lying states of Λ¹⁶O.Yiharn Tzeng, S. Y. Tsay Tzeng, T. T. S. Kuo, T.-S.H. Lee, and V. G. D. Stok

Angular correlation and coincidence studies of excited 0/sup +/ and other levels in the transitional Ce nuclides /sup 142/Ce, /sup 144/Ce, /sup 146/Ce, and /sup 148/Ce

The decays of the neutron-rich nuclides /sup 142/La, /sup 144/La, /sup 146/La and /sup 148/La to the levels of the transitional nuclides /sup 142/Ce, /sup 114/Ce, /sup 146/Ce and /sup 148/Ce, respectively, have been studied using mass-separated sources. Angular correlation studies have been employed to identify new excited 0/sup +/ levels in /sup 146 /Ce and support spin and parity assignments for 1/sup -/, 3/sup -/, 2 /sup +/, 3/sup +/, 4/sup +/ and 6/sup +/ levels in /sup 144/Ce and /sup 146/Ce. An excited 0/sup +/ level in /sup 148/Ce is indicated by coincidence ratios and electron studies, while no clear evidence can be found for the location of an excited 0/sup +/ level below 2 MeV in /sup 144/Ce. The resulting isotonic and isotopic systematics are discussed in terms of the transition from the closed shells at N=82 and Z=50 to the region of deformed nuclides. (13 refs)

Stochastic Thresholds: A Novel Explanation of Nonlinear Dose-Response Relationships for Stochastic Radiobiological Effects

New research data for low-dose, low-linear energy transfer (LET) radiation-induced, stochastic effects (mutations and neoplastic transformations) are modeled using the recently published NEOTRANS3 model. The model incorporates a protective, stochastic threshold (StoThresh) at low doses for activating cooperative protective processes considered to include presumptive p53-dependent, high-fidelity repair of nuclear DNA damage in competition with presumptive p53-dependent apoptosis and a novel presumptive p53-independent protective apoptosis mediated (PAM) process which selectively removes genomically compromised cells (mutants, neoplastic transformants, micronucleated cells, etc.). The protective StoThresh are considered to fall in a relatively narrow low-dose zone (Transition Zone A). Below Transition Zone A is the ultra-low-dose region where it is assumed that only low-fidelity DNA repair is activated along with presumably apoptosis. For this zone there is evidence for an increase in mutations with increases in dose. Just above Transition Zone A, a Zone of Maximal Protection (suppression of stochastic effects) arises and is attributed to maximal cooperation of high-fidelity, DNA repair/apoptosis and the PAM process. The width of the Zone of Maximal Protection depends on low-LET radiation dose rate and appears to depend on photon radiation energy. Just above the Zone of Maximal Protection is Transition Zone B, where deleterious StoThresh for preventing the PAM process fall. Just above Transition Zone B is a zone of moderate doses where complete inhibition of the PAM process appears to occur. However, for both Transition Zone B and the zone of complete inhibition of the PAM process, high-fidelity DNA repair/apoptosis are presumed to still operate. The indicated protective and deleterious StoThresh lead to nonlinear, hormetic-type dose-response relationships for low-LET radiation-induced mutations, neoplastic transformation and, presumably, also for cancer